1. Field of the Invention
The present invention relates to a capacitance type sensor suited for use in detecting a force and to a method for manufacturing same.
2. Description of Related Art
The capacitance type sensor generally is used as a device for detecting a force by converting a magnitude and direction of a force applied by an operator into an electric signal. Particularly, the recent utilization is as a two-dimensional or three-dimensional sensor that can detect an applied force on a directional-component basis. For example, there is an input device of cellular phone incorporating, as what is called a joystick, a capacitance type sensor to input the operation in multi-dimensional directions.
Meanwhile, the capacitance type sensor is allowed to input, as a magnitude of a force applied by the operator, an operation amount having a predetermined dynamic range. Particularly, the capacitance type force sensor, having a capacitance element formed by two opposed electrodes and for detecting a force based on a capacitance value change resulting from an electrode spacing change, is in practical application in a variety of fields because of its merit by virtue of simple structure and possible cost reduction.
Particularly, it is possible to consider a structure as shown, e.g. in FIGS. 39 and 40, for the use as a joystick on a cellular phone. In this capacitance type sensor 701, capacitance elements are constituted respectively between two kinds of opposed electrodes, i.e., fixed capacitance element electrodes E701-E705 and a displacement electrode 712 for displacement. An insulating film 713 is formed between the displacement electrode 712 and the capacitance element electrodes E701-E705. Meanwhile, as shown in FIG. 39, the capacitance type sensor 701 further possesses a substrate 720, a detection button 730 for the person to operate on the upper surface of the displacement electrode and externally apply a force thereon, a reference electrode (common electrode) E700 formed on the substrate 720, and a supporting member 760 fixingly supporting the detection button 730 and displacement electrode 712 on the substrate 720.
On the substrate 720, there are formed a capacitance element electrode E705 circular about an origin O, fan-shaped capacitance element electrodes E701-E704 outer thereof, and a reference electrode E700 further outer thereof and annular about the origin O, as shown in FIG. 40. Incidentally, signals including a clock signal are always inputted to the capacitance element electrodes E701-E705.
Now explanation is made on a method to detect a force by the capacitance type sensor 701. At first, in the case the detection button 730 undergoes an external force in Z-axis negative direction, the detection button 730 and the displacement electrode 712 displace together in Z-axis negative direction, to thereby change the spacing between the displacement electrode 712 and the capacitance element electrodes E701-E705. By the change in the electrode spacing, varied is the capacitance value of the capacitance element. Although the capacitance element electrodes E701-E705 are always inputted by signals as above, deviation in signal phase possibly takes place depending upon the change in capacitance value. Consequently, by utilizing such signal phase deviation, the force externally applied to the detection button 730 can be obtained based on the component in the X-axis, Y-axis or Z-axis direction.
According to the capacitance type sensor 701, the signal to the capacitance element electrodes E701-E705 is always inputted, i.e., not only upon operating the detection button 730 but also in the absence of operation. This results in wasteful consumption of power. As a method for reducing consumption power, there is a method that, when the detection button 730 is not operated for a predetermined time, a sleep mode is entered wherein signal input to the capacitance element electrodes E701-E705 is not allowed for power reduction to a possible low level whereas, when resuming the operation, the sleep mode is automatically canceled to resume the usual mode.
In order for automatic switching over between the usual mode and the sleep mode, it is a general practice to use an input device having a switch function of between on and off, together with a microcomputer control system. The output signal from the input device is a signal in a Hi-level at around a power voltage or in a Lo-level at around ground potential. The output signal, upon switching over, is turned from Lo-level to Hi-level or from Hi-level to Lo-level. Accordingly, in the input device, in the course of a transit from a non-operating state to an operating state, the output signal varies necessarily beyond a threshold voltage, a half of the power voltage. By monitoring the output signal, it is possible to securely detect an operation and correctly cancel the sleep mode. However, in the capacitance type sensor 701, there is a possibility that the output signal does not change beyond the threshold voltage depending upon a magnitude of a force applied to the detection button 730. Unless the output signal changes beyond the threshold voltage, it is impossible to securely detect an operation to the operation button 730 even when monitoring the output signal of the capacitance type sensor 701, resulting in a problem that the sleep mode is not to be properly canceled. Namely, there is a fear for the capacitance type sensor 701 not to properly effect a switching between the sleep mode and the usual mode, making it difficult to realize the reduction of consumption power.
By the mechanical nature of the displacement electrode 712 and under the influence of the mechanism supporting the displacement electrode 712, the displacement electrode 712 once deformed is not ready to restore fully the original position even when the force is released away, possibly suffering from some deviation at around the operation. Such deviation disadvantageously emerges as hysteresis on the output signal of the sensor. In the capacitance type sensor 701, there is always application of a voltage to the capacitance element constituted between the capacitance element electrodes E701-E705 and the displacement electrode 712 regardless of a presence or absence of operation to the detection button 730. Consequently, the charge stored on the capacitance elements is not negligible in amount even while the detection button 730 is not operated. The storage amount of charge on the capacitance elements is varied by operating the detection button 730. Because the amount even before operation is not negligible in magnitude, there is no sudden change in the course of a transit from an non-operating state to an operating state. Thus, where the change of charge amount is slight at around the operation, it is impossible to neglect a change of electrode spacing due to a positional deviation of the displacement electrode 712, increasing the hysteresis on the output signal.
Besides, the capacitance type sensor 701 can be suitably utilized as a device (force sensor) to recognize a force magnitude when the operator pushes down the detection button 730. However, it is not suited for the utilization as a device having a switch function for changing-over between different two states (e.g. on-state and off-state). Accordingly, where the capacitance type sensor 701 is built in an apparatus as a device having a switch function toward each direction, there is a difficulty in using the capacitance type sensor 701 as it is, requiring to separately provide a switch function corresponding to the directions.
Meanwhile, for manufacturing a capacitance type sensor 701, capacitance element electrodes E701-E705 and reference electrode E700 are arranged on a substrate 720 for example by printing and etching. After that, these electrodes E701-E705, E700 are covered by an insulating film 713, over which is provided a displacement electrode 712 of conductive rubber or the like. Then, a detection button 730 is set up and further the entire is fixed by a supporting member 760. Thus, a comparatively troublesome process is needed. Such a troublesome process is similarly required for the conventional other capacitance type sensors, besides the foregoing capacitance type sensor 701.